The present invention relates to an apparatus and a method for obtaining functional images of the retina, in particular, an apparatus and a method for obtaining, in vivo, a measurement of retinal response to a visual stimulus.
Electrophysiology studies the electrical properties of tissues and cells by measurement of changes in potential differences or of the flow of electrical currents therein.
In the field of ophthalmology, electroretinograms (ERGs) are used to measure the electrical response of the retina to visual stimuli. An ERG is a measurement of the electrical response of the retina to a full field sensory stimulus and is a triphasic waveform which carries information about the behaviour of various cell types in the retina. It is a composite global response generated by different cellular mechanisms, for example, those of the light sensitive rods and cones and the ganglion cells.
An ERG is obtained by placing electrodes on the cornea and on the skin adjacent to the eye, and measuring the electrical signal which is produced in response to a stimulus as a function of time. ERGs may be used in the diagnosis of retinal diseases such as retinitis pigmentosa and congenital stationary night blindness.
This method has a number of drawbacks. Firstly, it is generally uncomfortable to place electrodes on the cornea of a patient. Furthermore, because physical contact with the eye is necessary, the procedure must be carried out in a hospital environment and is therefore not a service suitable for general optical practitioners, such as high street opticians, to provide. Also, as the method uses a non-localised stimulus, the response measured is a bulk response of the retina as a whole from which it would be difficult to obtain accurate information about specific locations on the retina.
Multifocal ERG uses a number of distinct stimulus sequences to stimulate different regions of the retina simultaneously. As each sequence is distinct, the response for each region may be determined by cross-correlation of the known distinct stimulus sequences with a measurement from a single corneal electrode.
U.S. Pat. No. 4,846,567 describes a multifocal ERG technique in which identical, time-shifted, maximum length sequences (m-sequences) are used to visually stimulate the retina. Electrical signals are measured by means of an electrode placed on the eye, and the measured electrical signals correlated with the m-sequences to determine an electrical response for each m-sequence. The benefit of using m-sequences is that multiple time shifted versions of the same sequence can run simultaneously and enable the recovery of very small signals from multiple retinal sites.
However, this technique suffers in that it still requires the use of an electrode placed on the cornea to measure the electrical response.
It is therefore an object of the present invention to provide a method and an apparatus that obviates and mitigates one or more of the disadvantages and limitations of the prior art.